1. Field of the Invention
This invention is directed to electrical switching apparatus and, more particularly, to circuit interrupters, such as a circuit breakers, which respond to ground faults, and in particular to such circuit interrupters which are employed with downstream devices.
2. Background Information
Electrical switching apparatus include, for example, circuit switching devices and circuit interrupters such as circuit breakers, contactors, motor starters, motor controllers and other load controllers. Circuit breakers are generally old and well known in the art. An example of a circuit breaker is disclosed in U.S. Pat. No. 5,341,191. Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition.
Ground fault circuit interrupters include ground fault circuit breakers (GFCBs) and other ground fault contactors, motor starters, motor controllers and other load controllers.
Ground fault switches include ground fault receptacles (GFRs), and cord-mounted or plug-mounted ground fault protection devices (e.g., ground fault protection circuitry at the alternating current (AC) plug end of the AC power cord of an appliance, such as a hair dryer).
A typical ground fault circuit interrupter includes an operational amplifier which amplifies a sensed ground fault signal and applies the amplified signal to a window comparator. The window comparator compares the amplified signal to positive and negative reference values. If either reference value is exceeded in magnitude, a trip signal is generated.
A common type of ground fault detection circuit is the dormant oscillator detector. This detector includes a first sensor coil through which the line and neutral conductors of the protected circuit pass. The output of the first sensor coil is applied through a coupling capacitor to the above-described operational amplifier followed by the window comparator. A line-to-ground fault causes the magnitude of the amplified signal to exceed the magnitude of one of the reference values and, thus, generates a trip signal. The dormant oscillator ground fault detector also includes a second sensor coil through which at least the neutral conductor passes. A neutral-to-ground fault couples the two detector coils causing the amplifier to oscillate which also results in the generation of the trip signal.
GFCBs can provide an additional level of wiring protection, especially when used in conjunction with arc fault detection (AFD). However, there has been a tendency to favor the use of GFRs over the use of GFCBs, due to the convenience of localized resetting. GFRs cannot, of course, protect any wiring preceding them.
Accordingly, there is room for improvement in the circuit interruption function of electrical switching apparatus.